1. Field of the Invention
The present invention relates generally to packaging materials which enhance freshness by removing hydrogen sulfide and other odorous sulfur products from certain foods and non-food protein sources. More specifically, the packaging materials of the present invention comprise a defined molecular sieve enclosed by a polymer film or sheet, or embedded within a polymer film or sheet, particularly when the film or sheet is oriented, and even more particularly when it is also heat-shrinkable. The packaging materials may be used as a packaging structure (i.e., a package itself such as a film package), a layer of a film package, or a package insert such as a coupon or liner.
2. Description of Related Art
In food packaging, the principle emphasis in relation to odor removal has been with the packaging material itself. Many polymeric packaging materials produce odors which can impart an undesirable odor to the food. Various additives have therefore been added to the polymer to remove its odor. However food itself, even while it remains in good condition, can produce undesirable odors, and some of the odor producing substances have such a low odor threshold that trace mounts are undesirable. In addition, trace amounts of such substances can produce an off-taste in food.
Sulfur compounds having undesirable odor can originate from proteins and free amino acids in foodstuffs. They may originate from sulfur-containing amino acids of fresh, cooked or preserved foods including meats particularly poultry, cheese, dairy products, eggs and ovalbumin. Among these are sulfur-containing compounds, including hydrogen sulfide, mercaptans, sulfides and disulfides. These sulfur compounds can discolor cans containing such food, and there has been an abundance of work directed to can coatings to prevent this discoloration. However, a more important aspect is the presence of the odor, and often an associated taste, in the food itself. The odor threshold of some of these is very low (10 micrograms/liter or less) so that even small amounts can, as noted, be very objectionable well before any real deterioration of the food has occurred.
Packaging materials are commonly polymeric films. These may be essentially unoriented, or oriented in either one or two directions. Oriented films may also be heat shrinkable. Orientation of films may be used merely to improved mechanical properties. There is a vast patent literature of preparation of oriented films. Orientation is carried out in controlled temperature ranges typically well below melt processing temperatures, and commonly near the melting point or glass transition temperature, where molecular orientation occurs and the film does not tear. The temperatures allow sufficient mobility to orient, yet sufficient lack of fluidity to hold the orientation until the film is cooled or quenched. Oriented films may be annealed to reduce or remove any heat-shrinkable nature, or they may be oriented and subsequently treated in such a way as to provide and maintain a heat-shrinkable nature. Typically, the film is cooled in the stretched state and upon subsequent heating, the film shrinks in seeking to recover to its original dimensional state.
U.S. Pat. No. 3,141,912 describes a process for making heat shrinkable film that has achieved considerable commercial utility. Here the polymer is extruded into a film tube which is then quenched to a temperature below the orientation temperature range, and is reheated to the orientation temperature range, followed by biaxially stretching of the film of the tube while within this temperature range. The biaxial stretch is done by (a) using internal gas pressure to expand the diameter of the tube to form a large `bubble` and (b) advancing the expanded tube at a faster rate than the extrusion rate so as to obtain transverse and machine direction orientation. The patent is hereby incorporated by reference.
U.S. Pat. No. 4,820,557 discloses the manufacture of multilayer heat shrinkable film in which one of the layers is a linear copolymer of ethylene with either 1-octene or 1-hexene.
U.S. Pat. No. 4,597,920 discloses the manufacture of heat shrinkable film by the process of U.S. Pat. No. 3,141,912 wherein the linear copolymer is of ethylene with at least one C8-C18 alpha-olefin. The commercial shrink film of ethylene/1-octene of this process is called Clysar.RTM. LLP shrink film. The U.S. Pat. No. 4,597,920 is hereby incorporated by reference.
Oriented, and particularly heat-shrinkable film can be particularly adaptable to packaging, including food packaging. Continuous packaging machines which pack chicken parts in a tray, for instance, may advantageously employ shrink film.
Many materials can remove sulfides by reaction with them. Metal oxides, particularly zinc oxide have been used. Industrially, removal of sulfides in various processes is often carried out by reaction. In general, using chemical reactions to remove sulfides is fine in chemical processes, but not very suitable for food use, both because of toxicity concerns and the difficulty of controlling any such process. Adsorption is an alternative to reaction. Many adsorbers are unsuitable for contact with food. However, while several adsorbents may be entirely suitable for contact with food, there appears to be no work directed specifically to removing trace amounts of sulfur-compound related odors from food with their use.
Packaging materials capable of scavenging specific decomposition products from food have been disclosed in U.S. Pat. No. 5,284,892 (Brodie and Visioli). Specifically, aldehydes are scavenged by a polyalkylene imine of greater than 800 molecular weight. The imine is incorporated into a packaging film or sheet. Similar scavengers are disclosed in allowed patent application, Ser. No. 08/176,740, which extends their use to packaging structures which include liners, inserts and the like. Polyalkylene imines are unsuitable for direct food contact, and so when used, the required packaging structure has to ensure that the imine does not contact the food.
Molecular sieves are materials of varying composition which have a three dimensional cage like structure which form channels with defined mouth opening (aperture or pore-opening) sizes. They may be naturally occurring materials, modified natural materials, or essentially synthetic. They include silicas, metalloaluminates, aluminophosphates and various others. The silicas may by subdivided into titanosilicates, aluminosilicates, gallosilicates, ferrisilicates, borosilicates, chromosilicates and others. Among the aluminosilicates are the zeolites. Specific zeolites have been used for different purposes including catalysis, sieving and adsorption. Typically, zeolites are defined by parameters such as Si/Al ratio, their pore-opening size and structure, acidity, the cations present and others.
Clays may also be aluminosilicates also containing magnesium, but typically have a laminar structure, not a three dimensional structure.
Japanese published application, J61,120,638A discloses an adsorbent composite for packaging consisting of a polyolefin related resin and an adsorbent. The adsorbent is broadly disclosed to be any of a diverse range of materials including silica gel, activated alumina, acid clay, activated clay, zeolites or active carbon. Uses include dehumidifying and deodorizing pharmaceuticals, foods, or precision machines.
U.S. Pat. No. 4,795,482 (Gioffre et at.) discloses a process for removing odors from an `environment`, using certain specific hydrophobic, high silica zeolites and so-called silica polymorphs. Odors removable include those caused by a vast range of volatile materials including certain acids, aldehydes, organic nitrogen contain compounds and sulfur compounds including mercaptans and sulfides. Applications include powders, sprays, pads, creams, mouthwash, and non-woven tissues for treatment of odors in bathrooms, kitchens, refrigerators etc.
U.S. Pat. No. 5,011,019 (Satoh et at.) discloses a packaging structure for packaging medicines which may emit foul odors caused by substances such as carbonic acid, trimethylacetic acid and hydrogen sulfide. The package enclosing the medicine has a metal press-through side, and a multilayer polymer side, one layer of which is an polyolefin layer which incorporates deodorizing materials. The deodorizing materials mentioned as suitable include inorganic metal salts, favonoid, and unspecified molecular sieves.
There remains a need in the art for a packaging material, particularly in the form of oriented film which may also be heat-shrinkable, which can be used safely in contact with food, used either as a package itself, or as a packaging insert, specifically designed to remove hydrogen sulfide, mercaptans, sulfides and other odorous sulfur compounds from the specific foods which generate this odor, thus increasing shelf life.